Critical Susceptibility Exponent Measured from Fe/W(110) Bilayers

TL;DR

This study measures the critical susceptibility exponent in Fe/W(110) ultrathin films using an objective fitting method, finding results consistent with the 2D Ising model and exploring deviations near the critical temperature.

Contribution

Introduces an unbiased fitting approach to accurately determine the critical exponent gamma in ultrathin ferromagnetic films, confirming theoretical predictions.

Findings

Measured gamma ≈ 1.75, consistent with 2D Ising model

Objective fitting reduces systematic errors in critical exponent estimation

Deviations observed in films with limited power-law scaling range

Abstract

The critical phase transition in ferromagnetic ultrathin Fe/W(110) films has been studied using the magnetic ac susceptibility. A statistically objective, unconstrained fitting of the susceptibility is used to extract values for the critical exponent (gamma), the critical temperature Tc, the critical amplitude (chi_o) and the range of temperature that exhibits power-law behaviour. A fitting algorithm was used to simultaneously minimize the statistical variance of a power law fit to individual experimental measurements of chi(T). This avoids systematic errors and generates objective fitting results. An ensemble of 25 measurements on many different films are analyzed. Those which permit an extended fitting range in reduced temperature lower than approximately .00475 give an average value gamma=1.76+-0.01. Bilayer films give a weighted average value of gamma = 1.75+-0.02. These results are in agreement with the -dimensional Ising exponent gamma= 7/4. Measurements that do not exhibit power-law scaling as close to Tc (especially films of thickness 1.75ML) show a value of gamma higher than the Ising value. Several possibilities are considered to account for this behaviour.